This invention relates to extruding machines and more particularly to a machine for extruding hollow core sections from extrudable materials of which concrete is an example. For convenience, reference will be made hereinafter to articles formed of concrete, but without limitation thereto since the same principles of the invention may be applied to other materials which can be similarly handled.
Concrete sections or slabs having a plurality of hollow cores running longitudinally therethrough have become extremely popular in the building industry. Prestressed, precast concrete slabs may be used for floor and roof sections and are usually cast in lengths to be fitted together at a particular job site. These slab sections can also be used to form walls whereupon one slab section is stacked upon another and their end sections joined together. When these slabs are utilized in such building construction, one of the main virtues resides in the hollow core. This hollow core not only reduces weight but also provides chambers or ducts for air conditioning, wiring, plumbing and the like.
Machines for extruding a hollow core concrete slab section are well known in the art, examples of which are as follows: U.S. Pat. Nos. 3,284,867; 3,159,897; 1,445,724; Canadian Pat. No. 910,030 and Australian Pat. No. 228,818. Of these, all but one, i.e. Canadian Pat. No. 910,030, teach the production of concrete slabs with only a circular core defined therein. The Canadian Patent, on the other hand, as well as the present invention, are intended to permit cores of any desired cross section to be formed within the concrete slab. This is accomplished by the prior art, including the above referred to Canadian patent, by utilizing an initial auger section the shaft or core portion of which is defined by a surface of revolution such as a cone, with the outer surface of the auger being defined by a cylindrical envelope. At the rear portion of this initial auger section is a final trowel member which, since it is stationary and non-rotatable can be of any desired cross section. Thus, in the process of forming the concrete slabs the auger section forces the concrete rearward over the final trowel member causing the core formed in the concrete slab to assume the cross sectional shape of the trowel member, such as one having a rectangular cross section.
Regardless of which of the two devices for forming cores in concrete slabs are used, they both have the same deficiency in that during the operation of the machines for producing the slabs a build up occurs on the initial aguer section along the sides of the flights of the auger and the central portion forming the core thereof. This build up reduces the efficiency of the machine by reducing the volume rate of flow through the machine, thus, undesirably varying the pressure applied in packing the concrete as the slab proceeds off of the rear of the final trowel member. This difficulty can result in defective formation of the concrete slabs as well as a decrease in the efficiency of the apparatus. Also, it has been noted that when the machines are shut down for any repairs or changes it is necessary to remove the auger sections completely clean them to remove this build up, this being a very costly and time consuming task.